Automotive fluids, such as engine oil or transmission fluids, absorb heat in use. To prevent fluid deterioration, this heat often needs to be removed. Heat exchangers are commonly used for this purpose. Moreover, heat exchangers are known to perform this function adequately in moderate ambient conditions. However, in cold ambient conditions, engine oils and transmission fluids can be highly viscous. In such conditions, the flow resistance through the heat exchangers is very high. As a result, in such conditions, the flow in an oil circuit containing a heat exchanger can be impeded to such an extent that starvation of some downstream components, like transmissions, may even occur.
In order to avoid these adverse effects, it is known to provide a mechanism for bypassing the heat exchanger. One way that this has been done in the past is to provide a bypass conduit. The bypass conduit is connected in parallel with the heat exchanger and has a relatively low resistance to the flow of high viscosity fluids as compared to the heat exchanger. Structures of this type are known to avoid starvation of downstream components, but can suffer in that, in normal operating conditions, the flow is split between the heat exchanger and the bypass circuit. This requires that the heat exchangers be made proportionately larger and heavier to achieve the same overall heat exchange performance for the cooling system. This added size and weight, and the added costs associated therewith, are undesirable to automotive manufacturers.
To ameliorate the split-flow problem, it is known in the prior art to provide bypass valves. Sometimes, these bypass valves are pressure-activated, and are built into the heat exchanger. A heat exchanger exemplary of the foregoing is shown in U.S. Pat. No. 5,499,675 (Haasch et al.), issued Mar. 19, 1996. This structure includes a flapper valve of spring steel biased in a closed position, to prevent bypass flow, and which is adapted to: be urged, open when the fluid pressure inside the heat exchanger exceeds a certain limit, as indicative, of cold-start conditions. Heat exchangers of this general type can avoid starvation of downstream lubricated components, and can be adapted such that bypass flow is substantially nil in normal operating conditions, thereby to permit compact heat exchanger construction. However, in Haasch et al, the flapper valve is rather delicate and exposed while the heat exchanger is being mounted to the engine block, using an extension of the oil return pipe. The flapper valve is prone to suffering damage or being dislodged during installation. Also, heat exchangers of this type cannot be modified easily to accommodate different mounting or performance requirements in modern automotive applications.
It is also known to provide heat exchangers including a domed filter plate and a snap-in valve clip. Structures of this type are described in U.S. Pat. No. 4,561,494 (Frost), issued Dec. 31, 1985; U.S. Pat. No. 5,588,485 (Gire), issued Dec. 31, 1996; and U.S. Pat. No. 5,765,632 (Gire), issued Jun. 16, 1998. While the flapper valves in these structures are less prone to damage or dislodgement during heat exchanger installation, these heat exchangers are relatively inflexible in terms of the location of the bypass apertures or the size or shape of the oil filter that can be used with them.